Manual slideshow not working as intended

I am working on revamping a webpage for a local montessori. I am attempting to use the manual slideshow script, however, the page is not working as intended. Without adding an "onload" attribute, every image in the slideshow is displayed at once. Upon inspection, I have an error stating: "Uncaught TypeError: Cannot read property 'style' of undefined". Here is the HTML section in question:

The reason why slides[slideIndex-1] would be undefined is if slideIndex - 1 is beyond the bounds of slides. The most likely cause is that slides is empty. This could occur if you're running your script before the page content instead of after. This would cause this line to make slides length zero, since there are not yet any elements with the class name mySlides:

var slides = document.getElementsByClassName("mySlides");

Moving your script tag after the slide content should resolve the issue, or moving the showSlides(slideIndex) call in an onload handler.